The internal structure of turbine blades, internal combustion engines, electrical motors, household or industrial heaters or any heat conducting body having a internal flow circuit may be determined non-destructively and non-intrusively by means of heat transfer measurements. Such measurements allow inspection and quality control of the internal geometry and heat transfer parameters of the body where direct measurement is not possible.
In methods of the state of the art, measured temperature data of a body to be inspected is compared to data available from a body serving as a standard with known geometry and heat transfer characteristics. In further methods, the determination of heat transfer coefficients of the body to be inspected requires prior knowledge of the internal geometry and material properties.
In U.S. Pat. No. 3,566,669 to Lawrence a heating of cooling fluid is passed through a body, such as a turbine blade, whose internal structure is to be determined. A temperature pattern resulting on its external surface is measured and compared to the pattern of a standard body of known wall thicknesses and subjected to the same fluid flow. An obstruction in a cooling passage may be determined by comparison of adjacent exterior areas of the body.
U.S. Pat. No. 6,422,743 to Nirmalan discloses a non-destructive method for quantitatively determining heat transfer characteristics of a cooled structure. The method includes an infrared thermal imaging of the surface of a body that has been heated and through which a cooling medium is passed. Transient surface temperature data is acquired in pixel format using an infrared thermal imaging device and converted into a format including time, two-dimensional geometric position, and temperature. Heat transfer coefficients are obtained for each pixel on the surface of the body and for each time increment by processing the data using a numerical solution of a transient heat balance equation and taking a time average of the coefficient.
While this method allows the determination of heat transfer coefficients, it does not provide means to determine internal geometrical parameters of the body. An evaluation of the quality of a body is assessed by a comparison to a known body of its kind.
In the article by Nirm V. Nirmalan et al., “The measurement of full-surface internal heat transfer coefficients for turbine airfoils using a non-destructive thermal inertia technique”, Journal of Turbomachinery, Vol. 125, p. 83, January 2003, discloses a further method for determining heat transfer coefficients by means of thermal surface imaging. A body having internal flow structure is transiently heated or cooled by a heating or cooling medium of known temperature and flow rate. A two-dimensional pixelised infrared camera records the external surface temperature as a function of time. This transient thermal history contains the response data according to the internal heat transfer coefficients.
The internal heat transfer coefficient distribution is obtained by an iterative method that predicts the external surface temperatures using a finite element model of finite volume model and compares them to the measured temperatures. In case of insufficient fit to the measured values, the heat transfer coefficient is recalculated using an updated model. In the calculation of the heat transfer coefficients it is assumed that the walls of the internal structure conduct heat in one dimension only and heat conduction in lateral directions are negligible. Furthermore, the prediction of the external surface temperature requires known values of the body's internal wall thicknesses and structure geometry, material properties, and temperatures of the transient fluid. The method is therefore limited to bodies, for which these values are already known or which are determined by other methods.
The described methods are limited to the measurement of bodies for which the geometry of the internal structure is known or that is determined by another non-destructive method.